VARIABLE SPEED ELECTRIC DRIVE OF CENTRIFUGAL PUMP IN OIL LIFTING PLANTS

Abstract

Relevance of the work is caused by the need to create reliable frequency-controlled variable speed asynchronous induction motor drives of centrifugal pumping units for oil lifting. The main aim of the research is to study the frequency-controlled electric drives of centrifugal pumping units for oil lifting that operate at a speed higher than the nominal (rated) one. Methods of the research are based on the use of the automatic control theory and simulation in the MatLab – Simulink software environment. Results. The authors have made up the block scheme for simulation of a scalar frequency-controlled asynchronous induction motor drive of centrifugal pumping units for oil lifting. The scheme takes into account the main features of the real electric drive. The authors calculated mechanical characteristics of the electric drive of the pumping unit and determined the accessible ranges for controlling the speed of the electric drive of the pump unit based on the allowable power losses in steady-state and transient modes. Findings. Despite the fact that the static mechanical characteristic of an electric drive with ac converter frequency of 30 Hz is the minimum possible for oil lifting using submersible pumps, the range of speed control in an asynchronous induction motor drive of a submersible pump must be expanded to D=1:10 for a soft starting of the electric drive. It was found that in order to reduce high torque when starting an asynchronous frequency-controlled induction motor drive the frequency ramp-up rate at the output of the ramp generator should be several tens of seconds even for starting at the minimum operating speed of the electric drive. It was established that the inevitable power overrating of the induction motor for a submersible centrifugal pump, when it is selected as the closest higher one from a series of nominal values, allows the electric drive of the pump with frequency speed control to work without overheating on static characteristics above the nominal (rated) one. It was found that in frequency-controlled asynchronous induction motor drives the increase in the pump electric drive speed as a rule does not exceed 10–12 % of the nominal (rated) one, which allows increasing oil lifting output without upgrading the electric drive equipment.

Keywords

• centrifugal pumping unit
• induction motor
• indirect ac power frequency converter
• voltage inverter
• scalar control
• energy loss